1. Field of the Invention
The present invention relates to an improvement for a hydraulic pressure control device for a continuously variable transmission. More specifically, the present invention relates to a hydraulic pressure control device that controls hydraulic pressure of a clutch supplying torque to a continuously variable transmission while the vehicle is stopped and in a running range.
2. Background Information
In conventional belt-type continuously variable transmissions, there is a technique for carrying out a so-called idle neutral control for improving standing time fuel consumption. When using the so-called idle neutral control, a forward or reverse clutch and a forward/reverse switching clutch are released to create a state similar to when the shift range is in the neutral range, and the drive load of the engine is reduced to achieve idle rotation during a vehicle stop in a state in which the shift range is maintained in the drive range or another running range. An example of the so-called idle neutral control is disclosed in Japanese Laid-Open Patent Application No. 2002-295529.
In a conventional idle neutral control, when the driver attempts to start moving from the idle neutral control, it is necessary to immediately engage a forward clutch or the like and create a state in which movement can be started. The clutch pressure in the idle neutral state must be accurately controlled and kept at a prescribed clutch pressure that is slightly lower than the pressure at which the clutch is engaged.
If the clutch pressure cannot be controlled and kept at this prescribed clutch pressure, there will be variations in the hydraulic system during idle neutral control, variations in the accuracy of the clutch components, and installation error variations. Thus, either resulting in a continuously variable transmission in which the clutch cannot be expected to maintain an engaged state and fuel consumption cannot be expected to be improved, or resulting in a continuously variable transmission that has reduced start-up performance in which the clutch pressure is reduced and time is required to start the movement.
Consequently, Japanese Laid-Open Patent Application No. 2002-295529 discloses a method in which a torque transmission point at which the clutch is engaged and torque begins to be transmitted is determined by learning from the prior engagement of the clutch. Specifically, a comparison is made between the clutch input rotational speed and the engine speed when the clutch is gradually engaged, and the clutch supply hydraulic pressure at the time that the clutch input rotational speed is lower than the engine speed by a prescribed rotational speed is determined to be the torque transmission point. The clutch supply hydraulic pressure is controlled and the idle neutral control is carried out so that the learned torque transmission point is maintained when the vehicle is stopped in the D range.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved hydraulic pressure control device for a continuously variable transmission. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.